1. Field of the Invention
The present invention relates to telephony signaling, and more particularly, to a device for translating SONET-formatted signaling to a signaling format wherein the signaling bits for each data channel can be associated with their corresponding data.
2. Description of the Prior Art
The American National Standard Institute, Inc. (ANSI) T1.105-1988 describes the Synchronous Optical Network (SONET) protocol for telecommunications equipment. This standard is incorporated herein by reference. The SONET protocol is particularly adapted for optical transmission, and various transmission levels have been standardized at specified line rates in M bit/s. The first level, Optical Carrier Level 1, or OC-1, transmits data at the rate of 51.84 M bits/s. This carrier level has a corresponding electrical level called Synchronous Transport Signal Level 1, or STS-1.
In order to access this high-frequency carrier level, access products are required so that lower bandwidth carriers can be introduced into or extracted from the STS-1 transmission level. These access products provide a SONET network with nodes where components of an STS-1 signal can be added to or dropped out of the main signal. The components that are extracted must be reorganized to produce a signal compatible with currentlyused telephony standards. A typical sub-component of an STS-1 signal would be a DS1 signal having a bit rate of 1.544 M bits/s. Twenty-eight DS1 signals can be supported by an STS-1 carrier. Within the DS1 signal format, an additional 24 DSO 64K bits/s signals can be supported.
The SONET transmission is serial, comprising a total of 810 bytes. The frame structure for an STS-1 is shown in FIG. 1. The frame comprises 90 columns .times.9 rows of bytes, with 8 bits per byte. The sequence of transmission of the bytes is row by row, from left to right. The frame is divided into three parts: the section and line overhead, which are contained in the first three columns; and the payload, which is found in the 87 remaining columns, which, in connection with the nine rows, forms a Synchronous Payload Envelope, SPE, which includes 783 bytes. The information within the SPE is transported in sub-STS-1 payloads called Virtual Tributaries, or VTs. There are several levels of VTs; however, it is only necessary to deal with a VT 1.5 for purposes of this invention. When the STS-1 payload supports 28 DS1 services, one VT at the 1.5 level is provided for each DS1 service.
FIG. 2 illustrates the payload mapping of bytes into a DS1.
An SPE consists of 783 bytes belonging to 28 tributaries, wherein each tributary can carry a DS1 payload, as illustrated in FIG. 2. A DS1 payload has 27 bytes, 24 of which carry DS0 channels. The first byte carries a VT pointer, or address; a second byte is unused; and the third byte carries signaling data for the DS1 payload, which data is relevant to the DS0 channels carried in the DS1 payload. Every channel has four signaling bits, namely A, B, C and D, as is well known in the telephony art. Thus, for a DS1 payload of 24 channels, a total of 96 signaling bits are required. Since only four bits of signaling are carried in each signaling byte and there is only one signaling byte per tributary or DS1, a total of 24 SONET frames would be required in order to provide the 96 required signaling bits.
FIG. 3 illustrates the transmission order of the payload contained within the SPEs of 24 SONET frames. For the sake of clarity, the first two rows of each SPE, which would contain bytes 1 and 2, as shown in FIG. 2, for each of the 28 tributaries, have been omitted. This facilitates the illustration in FIG. 3 of the signaling bits provided in each SPE byte. Thus, the first row of each frame shown in FIG. 3 is the signaling row and contains byte number 3 for each of the tributaries. The transmission order proceeds from left to right in each descending row of a frame. Thus, the signaling bits for tributaries 0-27 are first transmitted in sequence, after which the data for channels 0 for each tributary followed by the other channels through to the transmission of the data for channels 23.
Due to the presence of nine overhead bytes, not shown, bytes 1 and 2 of each tributary, and additional unused `fixed stuff` bytes in the SPE, the signaling bytes start from SPE byte 60 and continue through byte 87. The content of each signaling byte is as follows:
______________________________________ (MSB) (LSB) Bit No. 7 6 5 4 3 2 1 0 ______________________________________ Byte Sync R R S1 S2 S3 S4 F R Bit Sync 1 0 R R R R F R R bits are not used ______________________________________
In the above, S1, S2, S3 and S4 are the signaling bits corresponding to the sets of four bits shown in the signaling bytes in FIG. 3.
Thus, the signaling bits received in the SONET signaling row are received in the order of A bits, B bits, C bits and D bits, which bits are not easily associated with their corresponding channel data. Thus, a system was required to re-associate the signaling with the proper channels and to store the signaling so that it is readily accessible during any internal system byte time. Thus, a storage of 2,688 signaling bits was required, with the bits arriving as shown in FIG. 3, while the storage was required to have an output format wherein the A,B,C and D bits of each channel could be accessed simultaneously without difficulty. Such a storage requirement created significant design problems, due to the large amount of storage required and the incompatible nature of the write bit addressing requirements and the read bit addressing requirements. If such a design were implemented in a full custom RAM, with independent bit write address and byte read address, design costs and schedule impacts would have been unacceptable.
It would have been possible to design the function using latches and independent read and write address decoders, but the latches alone would have consumed a significant amount of the limited surface area in the semi-conductor devices used to implement the system.